The in vivo fluorescence yield of chlorophyll a in Chlorella pyrenoidosa is shown to vary with the availability of nitrate, trace elements, and with the presence of mercury. The fluorescence yield of Phaeodactylum tricornutum also varies with culture conditions and comparisons of Cyclotella nana and Chaetoceros galvestonensis show a species‐dependent variation. Both types of yield fluctuations are eliminated when photosynthetic electron transport is blocked by adding 3‐(3,4dichlorophenyl)‐1,1‐dimethylurea (DCMU) to phytoplankton: the in vivo fluorescence yield becomes maximal and a constant function of cellular chlorophyll a, regardless of growth conditions or of the species examined. Preliminary observations of samples from the Caribbean Sea also indicate a constant in vivo fluorescence yield in the presence of DCMU.

SummaryIn experiments with one freshwater (Chlorella pyrenoidosa) and three marine organisms (Phaeodactylum tricornutum, Cyclotella mna, and Chaetoceros galvestmensis), mercury was more toxic than the other metals tested (silver, cadmium, lead, and copper) ; and its toxicity is comparatively irreversible. Growth was monitored by changes in fluorescence of the cultures over a 3-day test period. The toxicity of the mercury varied inversely with the concentrations of nutrients present. Preliminary experiments indicate that mercury in the form of mercuric chloride is more toxic than as dimethylmercury.

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The effects of four trichothecene mycotoxins on carbon dioxide production by a yeast, Kluyveromyces marxianus, were examined. Carbon dioxide release rate was inhibited by 10% in less than 25 min or 50% in less than 65 min by 0.0625, 0.125, 1.25, and 12.5 μg/mL of verrucarin A, roridin A, T-2 toxin, and HT-2 toxin, respectively. Lower concentrations did not affect carbon dioxide release rate within 160 min. Individually, neither 0.625 μg/mL T-2 toxin nor 0.000125 μg/mL roridin A alters carbon dioxide production. When combined at these concentrations, the two trichothecenes interact synergistically to inhibit carbon dioxide release rate by 50% within 60 min. In the case of grains contaminated with mycotoxin mixtures this synergistic interaction may raise concerns about safe-exposure conditions. Key words: carbon dioxide, Kluyveromyces marxianus, synergism, trichothecene mycotoxins.

A laboratory evaluation has been made of water-soluble biocides which might be effective in controlling microbial contamination in water-compensated fuel storage tanks on naval ships. Higher concentrations of biocides were generally required for control of sulfate-reducing bacteria in steel drums simulating ships' tanks than in test-tube scale assays. At least tenfold higher concentrations were necessary when biocide additions were made after, rather than before, bacterial growth had developed extensively. A mercaptopyridine biocide was effective at the lowest concentration followed by methylene bisthiocyanate and a triazine derivative; an organoboron compound was comparatively ineffective. Attention has also been given to the possible environmental impact of discharging biocide-treated water. An algal assay has been used to study the loss in toxicity of candidate biocides in seawater solution on exposure to sunlight. The mercaptopyridine biocide was particularly susceptible to photodegradation. Thus it may be possible to control microbial contamination indefinitely in a dark fuel tank with a biocide which photodegrades to nontoxic products when discharged into sunlit surface waters.